The present invention related to a single transverse mode type of surface-emitting laser and a method of fabrication thereof.
A surface-emitting laser is known in the art in which a resonator post is formed as a column from an upper mirror and an embedding layer is formed around the periphery thereof. The resistance of this embedding layer is increased by ion implantation to ensure that current flows only through the resonator post.
However, the surface-emitting laser of this configuration emits laser beams in transverse modes that are controlled by the distribution of current, because there is substantially no difference in absolute refractive index (refractive index with respect to vacuum) between the resonator post and the embedding layer. In other words, a large proportion of the current flows through outer peripheral portions of the resonator post so that higher, ring-shaped transverse modes appear.
Furthermore, the other structure of the surface emitting laser has materials of low refractive indices, such as air, around the periphery of the resonator post so that a large difference in absolute refractive indices is created therebetween and thus the higher transverse modes are also enclosed therein. It is therefore impossible to cut out the higher transverse modes.
This makes it impossible for a conventional surface-emitting laser to achieve a laser beam that lases with a single transverse mode, particularly when a high level of power is required. It is therefore necessary to provide an optical system for focusing the laser beam when this surface-emitting laser is used in an optical information apparatus or the like.
The present invention was devised in order to solve the above problems of the prior art and thus provides a surface-emitting laser which is capable of emitting a laser beam with a single-peak spatial distribution, but is incapable of guiding modes other than a single transverse mode, and a method of fabrication thereof.
(1) A vertical-cavity surface-emitting laser in accordance with an aspect of the present invention comprises:
a columnar portion formed of at least part of a reflective mirror on a light-emitting side and an embedding layer surrounding the periphery of the columnar portion,
wherein the embedding layer has an absolute refractive index that is slightly smaller than that of the columnar portion.
This aspect of the invention also has an embedding layer surrounding the periphery of the columnar portion, and the absolute refractive index (refractive index with respect to vacuum) of the embedding layer is made to be smaller than the absolute refractive index of the columnar portion. This configuration makes it possible to ensure that all the light is totally reflected within the columnar portion and is enclosed thereby, in a manner similar to an optical fiber.
It is known that, when the radius of the core of an optical fiber and the difference in absolute refractive indexes between the core and cladding are large, a large number of modes can be transmitted thereby. Therefore, in order to transmit a single mode only, it is necessary to ensure that, when one of the radius and absolute refractive index of the core is large, the other one of them is correspondingly small. In a similar manner, it is necessary with the present invention to ensure that at least one of the radius of the columnar portion and the difference in absolute refractive indices is small, to transmit in a single transverse mode.
Furthermore, the difference in absolute refractive indices is made to be very small in this aspect of the invention, so that the radius of the columnar portion can be made as large as possible in correspondence thereto. The columnar portion is formed from at least part of a reflective mirror on a light-emitting side, so that if the radius of the columnar portion is made large, the light-emitting portion thereof is also large, and thus the optical output can be increased.
(2) With respect to the above described surface-emitting laser, it is preferable that:
a material forming the columnar portion is made to be single crystal; and
a material forming the embedding layer is the same material as that of the columnar portion, but is made to be non-single crystal.
The same material has a higher density and a higher absolute refractive index when it is made single crystalline, whereas it has a slightly lower density and a slightly lower absolute refractive index when it is made non-single crystalline (polycrystalline or non-crystalline). It is therefore possible to change the absolute refractive index slightly by making the material either single crystalline or non-single crystalline.
(3) With respect to the above described surface-emitting laser, the columnar portion may have a diameter of at least approximately 3 xcexcm; and
the difference in the absolute refractive index between the columnar portion and the embedding layer may be not more than approximately 0.01.
Making the diameter of the columnar portion at least approximately 3 xcexcm in this manner makes it possible to obtain a practicable laser beam output. In addition, to ensure transmission only in a single transverse mode when the diameter of the columnar portion is at least approximately 3 xcexcm, the absolute refractive index is made to be no more than approximately 0.01.
(4) It is preferable, relating to the above described surface-emitting laser, that:
the embedding layer has a low electrical resistance; and
an insulating layer is formed below the embedding layer.
Lowering the electrical resistance of the embedding layer in this manner makes it possible to restrain the generation of heat therein. In addition, the insulation layer ensures that no current flows below the embedding layer and thus the current is concentrated below the columnar portion, making it possible to obtain a high-power laser beam.
(5) A method of fabricating a vertical-cavity surface-emitting laser in accordance with another aspect of the present invention comprises the steps of:
forming a first single crystal layer at a position above an active layer but below a reflective mirror on a light-emitting side;
forming a non-single crystal layer on the first single crystal layer;
forming an aperture portion in part of the non-single crystal layer, to form an exposed portion of the first single crystal layer; and
growing a multi-layer film non-selectively on the non-single crystal layer that comprises the aperture portion,
wherein the multi-layer film is made to be non-single crystal on the multi-layer film and single crystal above the aperture portion.
With this aspect of the invention, the single crystal layer is exposed from the aperture portion, so that the multi-layer film that is grown by non-selective growth becomes a single crystal columnar portion above the aperture portion and a non-single crystal (polycrystalline or non-crystalline) embedding layer on the periphery thereof. Thus the above described surface-emitting laser can be fabricated in a simple manner.
(6) In relation to the above described method of fabricating a surface-emitting laser, it is preferable that:
the multi-layer film is grown after a second single crystal layer has been selectively grown only on the exposed portion of the first single crystal layer.
This ensures that the aperture portion of the non-single crystal layer is embedded with a single crystal layer and the surface of the exposed portion of the non-single crystal layer is flattened, so that the multi-layer film that is subsequently grown non-selectively thereon as the emitting-side mirror can be grown uniformly over a wide region.
(7) It is preferable, relating to the above described method of fabricating a surface-emitting-laser, that:
the growth of the multi-layer film is performed after the growth of an AlAs layer.
An AlAs layer grows easily in a non-selective manner on the non-single crystal layer and the single crystal layer, so that if such an AlAs layer is formed initially, it is easy to grow a multi-layer film of any composition thereafter.
(8) It is preferable, in relation to the above described method of fabricating a surface-emitting laser, that:
the multi-layer film has a low electrical resistance; and
the non-single crystal layer is an insulating film.
This makes it possible to form an insulating film below the region where part of the multi-layer film will form the embedding layer. Since the electrical resistance of the multi-layer film is lowered, the generation of heat in the embedding layer can be restrained, and the insulation film ensures that the current is concentrated below the columnar portion formed from the multi-layer film.